A bleaching procedure is generally understood as the chemical destruction of chromophores in organic or inorganic compounds; where the purpose of bleaching is to increase a weak brightening or to improve the color of the material that is bleached
At the industrial level, bleaching is often combined with cleaning agents. Scrubbing or rubbing a fabric through pre-bleaching means treating a fabric or a fiber with an alkali, pumice stone and enzymes at 65° C. Scrubbing main functions are to dissolve stains or particles to emulsify waxes and oils formed by the absorbents of the fabric to bleach. After scrubbing the fabric is ready to be bleach with sodium hypochlorite.
Historically, the sodium hypochlorite was first used in the fabrics industry to bleach linen. When the use of cotton was introduced, this fiber was also bleached with the hypochlorite. The treatment with hypochlorite is followed by a treatment of the anti-chlorine (sodium bisulphite, sulphur dioxide, or hydrogen peroxide) to avoid the color fading (cloramides formation). However, the hypochlorite solutions previously used for bleaching fabrics, cause the undesirable corrosion of the equipment. Furthermore, hypochlorite solutions products have low concentrations of HOCL and contain impurities that substantially reduce their stability. During the bleaching procedure, followed by the sizing material removal, the fabric is immersed in a sodium hypochlorite solution heated during a period of time long enough to remove stains or particles.
Then, the scrubbed fabric, is treated with the bleaching solutions, containing several additives such as silicates, at higher temperatures during long periods of time in order to bleach fabrics.
Bleaching compositions are used for a variety of several purposes, being particularly interesting herein, the fabrics bleaching with a solution containing sodium hypochlorite and a further bleacher comprising sodium hypochlorite, water, phosphates and phosphoric acid.
Frequently, fabrics bleaching procedure presents the inconvenience of yellowing the fabric, and this is due to the hypochlorite used for the bleaching procedure.
It has been found that this defect is directly related to, when the hypochlorite solutions used for bleaching the fabrics additionally comprise an effective amount of silicate metal alkaline salts, such as Cu, Fe, Ni, and Co. It is widely known that the presence of silicates within the solution plays the role of moderating or preventing the attack of the hypochlorite upon the brightening agents that are deposited upon the surface of the fabrics or clothes during their elaboration.
This is, in the absence of silicates, the hypochlorite attack occurs upon the brightening agents deposited over the fabrics surface, which causes the yellowing phenomenon.
In this field, bleaching compositions are known, and particularly bleaching compositions comprising phosphates and additionally said compositions also comprise pH buffering components, obtaining chemically stable compositions. By chemically stable compositions it is understood that the hypochlorite bleaching composition does not suffer any loss higher than the 15% of chlorine available after five days of storage at 50° C.±5° C.